Urea-ammonium polyphosphate production

ABSTRACT

Pugmill granulation process for the production of fertilizers containing urea and ammonium polyphosphate. Concentrated urea solution is sprayed onto recycled solids in the pugmill followed by molten ammonium polyphosphate. The melt is fed to the pugmill at a minimum of 12 inches downstream from the urea to prevent hydrolysis of the urea which results in foaming and gas evolution. The product from the pugmill is essentially anhydrous, and no further drying is required. Other fertilizer materials such as ammonium sulfate, potassium chloride, or micronutrient sources may be incorporated by adding them to the pugmill during granulation.

United States Patent 1 Lee et al.

[111 3,825,414 [4.5] July 23, 1974 UREA-AMMONIUM POLYPHOSPHATE PRODUCTION [75] Inventors: Robert G. Lee, Florence; Robert D.

Mitchell, Sheffield, both of Ala.

[73] Assignee: Tennesse Valley Authority, Muscle Shoals, Ala.

22 Filed: Dec. 10, 1973 21 Appl. No.: 423,419

Related U.S. Application Data [52] U.S. Cl. 71/29, 71/34 Bottai et al. 71/29 Legal et al 71/29 Primary ExaminerSamih N. Zaharna Assistant ExaminerRichard Barnes [57] ABSTRACT Pugmill granulation process for the production of fertilizers containing urea and ammonium polyphosphate. Concentrated urea solution is sprayed onto recycled solids in the pugmill followed by molten ammonium polyphosphate. The melt is fed to the pugmill at a minimum of 12 inches downstream from the urea to prevent hydrolysis of the urea which results in foaming and gas evolution. The product from the pugmill is esgg g lg sentially anhydrous, and no further drying is required. f I6 4 D Other fertilizer materials such as ammonium sulfate, potassium chloride, or micronutrient sources may be [56] References Cited incorporated by adding them to the pugmill during ranulation. 1 UNITED STATES PATENTS g 3,540,874 11/1970 Stinson 71/34 X 1 Claim, 1 Drawing Figure PHOSPHORIC conc ggrxArEo /Z ACID Petra 33st, 1 AMMONA REACTOR AMMONI M 3 POLYPHOSPHATE 7 MELT 2 RECYCLE FEEDER 4 PUGMILL RECYCLE FINES FLOWSHEET OF UREA+AMMONIUM POLYPHOSPl-lATE PILOT PLANT UREA-AMMONIUM POLYPHOSPHATE PRODUCTION This application is a continuation-in-part of our copending application Ser. No. 202,836, filed Nov. 29, 1971, now abandoned which, in turn, is a continuationin-part of application Ser. No. 130,236, filed Apr. 1, 1971, now Defensive Publication T900,0l8, both for Urea-Ammonium Polyphosphate Production.

The invention herein described may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty therefor.

Our invention relates to a newly developed process for the production of urea-ammonium polyphosphate granules, more particularly to the granulation of mol-' ten ammonium polyphosphate and concentrated urea solution in a pugmill or blunger type of granulator, still more particularly to the production of granular ureaammonium polyphosphate wherein the resulting product is in a highly crystallized form eminently suited for either short-term or long-term storage, and even still more particularly to a newly developed process for the production of urea-ammonium polyphosphate in highly crystallized form by mechanically working a molten mixture of ammonium polyphosphate and urea by shearing action of the pugmill paddles to impart energy.

necessary to effect crystallization of ammonium polyphosphate from the melt phase. Alternately, the urea can be added to the pugmill in solid form, but addition of the urea as a concentrated solution is preferable because the solution is normally less costly to produce than the solid prills or granules.

Although urea-ammonium polyphosphate fertilizers lend themselves to a great variety of grades, many of which are higher in plant nutrient concentration than generally heretofore obtainable, it has been found that in order to obtain urea-ammonium polyphosphate fertilizer material in a form well suited for storage and handling as well as application, it is necessary to have.

the product in a highly crystallized form rather than in amorphous form, which amorphous form has poor handling and storage properties. Storage tests have shown that the products we have produced by our process have excellent storage and handling properties.

Heretofore, one way to make crystalline ureaammonium polyphosphate is to add solid urea to molten ammonium polyphosphate and to oil prill the mixture. This process is described by Bottai and Stinson (US. Pat. No. 3,578,433 issued May ll, 1971). .In order to obtain homogeneous prills it was necessary to combine and mix the urea and ammonium polyphosphate melts prior to prilling. This combination of melts worked fairly well when only thermal phosphoric acid was utilized. However, when wet-process acid or mixtures of wet-process acid and thermal acid were tested in a pilot plant, the impurities in the wet-process acid resulted in an increase in the rate of hydrolysis. This hydrolysis resulted in loss of ammonia and foaming of the solution and melt mixture. Said foaming is at times so severe that it will spill over the top of a rotating prilling cup and can impart dissolved gases into the product prills. This problem of hydrolysis was so severe that it was a major factor in influencing us to abandon the oilprilling process in our planning for a production unit and causing us to search for a better process.

Work on our invention was initiated and inspired by an urgent need to develop a satisfactory commercial process for producing a urea-ammonium polyphosphate product in grannular or prilled form. We had intended to scale up into a full-size plant the process described by Bottai and Stinson, supra for production of urea-ammonium polyphosphate by oil prilling. However, subsequent pilot-plant work on a larger scale revealed that the Bottai et al process could not be satisfactorily utilized for the specific purposes intended for our plant operations.

The major problems we encountered in attempting to scale up the teachings of Bottai et al was severe and uncontrollable hydrolysis of urea when the urea and ammonium polyphosphate were mixed in their prilling cup. Bottai e't al did their work primarily with thermal acid which is of high purity but which has now become too costly'to use for fertilizer production. When we later tested their oil prilling process with wet-process phosphoric acid, which is the type acid that is normally used for commercial fertilizer production, the impurities in the acid catalyzed the hydrolysis of urea.

We also found that satisfactory centrifuging of the oil-prilled product to reduce the oil content to a r'easonably low level would require centrifuging equipment too large to be practical. We further realized that the capability for mechanical working, such as is afforded by a pugmill, is desirable if the plant is to be capable of producing a wide variety of urea and ammonium polyphosphate combinations.

Therefore, we began a search for a more satisfactory process that we could scale up toa full-size plant. This led to our invention of a new urea-ammonium polyphosphate process as described herein. Our new process will be used, rather than the Bottai process, for our production-size unit which is now under construction and is expected to be on stream by about Nov. 1, 1973.

In our initial work with a pugmill we attempted to combine the urea and ammonium polyphosphate melts in a trough which discharged onto recycle in the pugmill. Again, hydrolysis was so severe that we had to abandon any mixing of these streams and we resolved to feed the two streams separately into the pugmill. We then found that the points of addition of molten ammonium polyphosphate'and urea to the pugmill. can be so I separated that the hydrolysis reaction between the'hot ammonium polyphosphate melt and urea will not occur. Specifically, we found that spraying the hot urea solution onto recycle fines at a point separated by at least about 12 inches from the molten ammonium polyphosphate prevented hydrolysis. We consider that this is due to allowing some cooling of one of the feed materials in the pugmill before it contacts the other feed material. Such cooling prevents hydrolysis, which is promoted by high temperature.

We have developed a new process for the production of urea-ammonium polyphosphate material wherein the problems relating to decomposition and hydrolysis of urea that occur in the oil-prilling process are avoided. We have also avoided the costly and difficult problems of separating'oil from prilled products. This separation is normally made in a centrifuge, but large centrifuges for handling large rates of fertilizer production are not available, and the use of many small centrifuge units or centrifuges in parallel operation would be required for a commercial oil-prilling UAP process. Obviously, use of many small units in this way is highly objectionable. This problem with centrifuging. of oil was another reason why we abandoned our oil-prilling erage of results, particularly in the amount of urea feed process in favor of the pugmill granulation process for as dust that we have observed in the rest of our testing our production-scale plant. A survey of the literature procedures. From the tests we made in the pilot plant has revealed no patents pertaining to pugmill granulawe are m re than 95 rcent confident that if, for extion of urea and molten ammonium polyphosphate. 5 ample, a hundred such tests were made and listed as the Bottai and Stinson, supra, mention the probable retwo above for each condition, i.e., downstream and upquirement for meCha Ca ork g o apply gy of stream from the melt, the average urea feed recovered shear to satisfactorily granulate urea and ammonium as d t ld b i ht t b t 25 nt, wherein the polyphosphate melturea was fed downstream from the melt and right at In our early Work, We found that when urea and amabout 2 percent urea feed recovered as dust in the inmonium p lyph p melt were added at or y stances wherein the urea was fed upstream from. the near the same point in the pugmill, severe hydrolysis melt, it being understood, of course, that the relationreaetioh Occurred, Characterized by foaming and ship of the two feed streams be spaced at least about 12 hltioh of gases we then added the urea at a Point inches apart. For further purposes of illustration, the downstream from the Point of entry of melt into the 15 width of the pugmill used in these tests is 22 inches and P Adding the urea downstream from the molten the length about five feet and the streams. of feed were ammonium p yp p Prevented hydrolysis of directed onto the 'material covering the bottom of the urea, but the urea which was coated onto the outside pugmill in the general perpendicular attitude to the of the Particles dusted Off and was largely lost during bottom surface of the pugmill about half way between processing [It later tests the urea solution w p y the .peripheral edges thereof, i.e., about 11 inches in- Qhto the recycle material at 'h the Point of entry ward from the inner wall surface of the rim of said pugto the pugmill and the ammohlum polyphosphate was mill. The urea entered the pugmill at the upper end at added at a Polht P Substantially downstream or very near the point 'of entry of the recycle, with the and at least 12 inches o Point of urea. ehtryh melt being fed at a point about 12 inches'downstream ammonium polyphosphate melt overcoatedthe outside 25 from the point f urea entry of the urea and protected the urea from abrasive ac- It Seems Obvious f these data f 2 -2 4 grade tion, thereby preventing the dusting off of urea. This product that dusting ff f urea f the i l nrod method of positioning the feeds relative to each other net would iner'ease when making a product Containing hi the p g was entirely Satisfactory and Prevented a higher proportion of urea than 28-28-0, and less urea both chemical decomposition (hydrolysis) of the urea 30 would dust ff when making a pteduet Containing a feed and loss of urea due to dusting off the granule sursmaller proportion f urea than in 3-2 1 However, ee other factors affect dusting off of urea. The polyphos- By way of example of the effecti n f h adding phate content of the APP melt affects dusting off of of the urea upstream of the melt in the pugmill'in deurea because the stickiness of the melt, and thus the creasing the dustiness of the product, pilot-plant data bi di g perties, ar dir tly proportional t its lyare given. The operating conditions for two pilot-plant h hat t t Th r y l ati i e pounds f tests for production of 28-28-() grade urea-ammonium recycle per pound of product; used in the pugmill also polyphosphate wer essentially identical, e cep that affects dusting off of urea because it affects the particle the urea was added downstream from the ammon size distribution of the material in the pugmill. In subsepolyphosphate melt in test 142 and upstream from the 40 quent work, to minimize dustiness we ran tests of the melt in test I49. Results from these tests, tabulated beproduction of 28-28-0 and 34-17-0 grades with the low, illustrate the extreme urea dust formation when urea fed upstream from the APP melt. In these tests,

urea was added downstream from melt and the deletethe percentage of urea feed lost as dust ranged from rious effect on product grade. about 3 percent (at the highest polyphosphate level Chemical analysis, wt. Urea Test Location of urea Screen product Cyclone dust feed as No. feed entry Total N Total P 0 Total N Total P 0 dust,

I42 Downstream from 25.0 35.3 36.9 l4.4 About 25 melt I t 149 Upstream from 28.2 28.3 9.3 12.4 About 2 melt v In test 142, with urea added after the melt, product tested, 40 percent of the P 0 up to about 13 percent grade was 25-35-0, illustrating the severe loss of urea, for our highest urea grade product, to wit, 34-17-0. The the nitrogen component. An excessive amount of dust average dust loss for both 28-28-0 and 34-17-0 grades was collected in the cyclone dust collectors. This dust was about 6 percent. Comparing the results of these was 37-14-0 grade, evidencing the high urea content. tests with those shown .m our parent application wherein the urea feed percent as dust when location of In test 149, with urea sprayed onto recycle in the the urea entry was downstream from the urea melt pugmill upstream from the point of melt entry, th shows a decrease in the amount of urea dusted off rangproduct was exactly the 28-28-0 grade desired. Very ing from 500 percent to greater than 800 percent. lt little dust was collected in the cyclones, and analysisof should, of e, he un e oo that thls unprecethe d st indicat d that it c t in d no t i ht dented and drastic reduction in the severe loss of urea Th above t t 142 d 149 were i k d f is reported on the conservative sidc since it is composes of illustration because they reflect the general avpared Wlth the urea feed Percent duSt pared on the urea being downstream from the melt on a 28-28-0 grade. Still other tests wherein a direct comparison for each of the grades throughout our range was desired with the entry of the urea compared as downstream as against upstream observations and extrapolations indicated that in all situations the proper location of the urea feed, i.e., it being at least 12 inches upstream from the urea melt entry, could be expected to result in about a tenfold improvement in reducing the heretofore observed loss of urea as measured by the urea feed percent as dust collected.

. To the unenlightened, it would at first seem obvious that to avoid the problem of reaction of the urea with the APP melt, one would feed the melt first, thereby cooling it with the recycle and feeding the urea solution downstream from the melt, but this results in the problem of dusting off of urea as discussed above. We therefore found it necessary to feed the urea upstream and the hot APP melt onto the mixture of urea and recycle and it was in this arrangement wherein we discovered the unusual and unexpected result of no reaction between the hot APP melt and urea. Examples of this lack of reaction are given in tables I-A and ll-A below showing 18 percent of P as polyphosphate in the APP melt and the same 18 percent in the screened product for the 28-28-0 grade in test 207. Also, practically no reaction occurred in test 209 having 33 percent of the P 0 as polyphosphate in the APP melt and 34 percent as polyphosphate in the screened 19-19-19 grade prod- TABLE lI-A Pilot-Plant Granulation of Ammonium Phosphate Melt Product grade 28-28-0 l9-l9-l9 Test No. 207 209-2 Test length, hr. 5.0 4.5 Product rate. lb/hr. 1000 1480 Feed acid P 0 percent 50.7 51.0

Temp., "F 140 270 Rate, lb/hr.

Melt 500 500 Urea 500 500' Potassium chloride 480 Ammonium nitrate (97% soln.)

TABLE ll-A-Continued Pilot-Plant Granulation of Ammonium Phosphate Melt Product grade 28-28-0 19-19-19 Ammonium sulfate (crystalline) Recycle Ratio 3.9 3.0 Rate, lb./hr. 3900 4400 Temp, "F 104 122 Screen analysis, by wt.

+6 mesh 2 2 6 +10 mesh 27 24 -10 mesh 71 74 Pugmill slope, in./ft. 0.2 0.2 Pugmill product Temp., "F 186 176 Screen analysis, wt.%

+6 mesh 29 26 6 +10 mesh 31 30 1O mesh 40 44 Analysis of screened product (-6 +9 mesh), wt.%

N 28.1 20.1 P 0 30.6 194 K 0 19.1 H O 0.6 0.5 Percent of total P 0 Polyphosphate 18 34 Water soluble 98 98 Available 100 100 Urea solution, 99%. 280F.

It has been suggested by some that the rate at which the constituents pass through the pugmill relates to the time the recycle fines can cool the urea before admixture with the polyphosphate. We have looked into this aspect and have determined the residence time of the material in the pugmill as it travels from the point of entry of said urea to the point of entry of the polyphosphate melt some 12 inches downstream therefrom. We have determined that this residence time ranges from about one minute to about 7 minutes. For instance, the retention time in minutes is the volume of the pugmill in cubic feet divided by the throughput in cubic feet per minute. The volume of the pugmill is 19 cubic feet. The bulk density of the material averages 47.5 pounds per cubic foot. To arrive at throughput, any of the examples in our specification may be turned to. For example, table III infra with the wet-process materialv 28-28-0 shows feed rates of the melt and urea of 500 pounds per hour and that the recycle solids is 5800 pounds per hour, which totals 6800 pounds per hour throughput with a recycle ratio of 5.8. Sixty-eight hundred pounds per hour divided by 47.5 pounds per cubic foot equals 143 cubic feet per hour divided by 60 equals 2.38 cubic feet per minute. Plugging into the formula above for retention time, the volume divided by the throughout equals 19 divided by 2.38 equals 8 minutes. From table ,A, it may be seen (bottom line thereof) that the recycle ratio ranges from 0.5 to 10. Thus, x divided by 8 equals 5.8 1.0 divided by 0.5 1.0 equals 35 minutes maximum retention time and 5.8 divided by 10 1 equals x divided by 8 equals about 5 minutes minimum retention-time, these retention times of about 5 minutes to about 35 minutes being over the total effective length of 5 feet of the pugmill, thereby yielding a range of retention times for the 12 inches of separation between the points of entry of urea and ammonium polyphosphate from about 1 minute to about 7 minutes. Throughout equals pounds per hour recycle (5800) plus pounds per hour product (500 melt plus 500 urea).

We have found that our process has no limitations with regards to proportions of urea and ammonium polyphosphate melts that can be utilized. Products ranging from about 100 percent urea to about 100 percent crystallized ammonium polyphosphate melts can be granulated, whereas Bottai et a1; supra, were limited to oil prilling of products ,with a proportion of urea to ammonium polyphosphate in the range from about 85:15 to about 25:75.

It is therefore an object of the present invention to produce granules of highly crystallized ureaammonium polyphosphate which are in phase equilibrium and which therefore have excellent handling and storage properties.

Another object of the present invention is to produce granules of highly crystallized urea-ammonium polyphosphate which are in phase equilibrium and which therefore have excellent handling and storage properties by a process wherein the less expensive liquid form of urea may be utilized rather than the more expensive solid form of urea.

Still another object of the present invention is to produce granules of highly crystallized urea-ammonium polyphosphate which are in phase equilibrium and which therefore have excellent handling and storage properties by a process wherein the less expensive liquid form of ureamay be utilized rather than the more expensive solid form of urea, and wherein the previous difficulties of hydrolysis and decomposition of urea that occur when said urea is mixed directly with ammonium polyphosphate melt, as is done in the oil-prilling operation, are avoided, and dusting of urea from the product is prevented.

A further object of the present invention is to produce granules of highly crystallized urea-ammonium polyphosphate which are in phase equilibrium and which therefore have excellent handling and storage properties by a process wherein the less expensive liquid form of urea may be utilized rather than the more expensive solid form of urea, wherein the previous requirement for centrifuges for separating oil from oilprilled products is avoided, this being especially advantageous in large, commercial-scale operations in which a multiplicity of centrifuges in parallel operation would be required, whereas our invention requires but a single line of equipment.

Still another object of the present invention is to produce granules of highly crystallized urea-ammonium polyphosphate wherein the previous limitations of proportions of urea to ammonium polyphosphate melt are avoided because by our invention products ranging from about almost all urea to about almost all ammonium polyphosphate can be produced.

Still further and more general objects and advantages of the present invention will appear from the more detaileddescription set forth below, it being understood, however, that this more detailed description is given by way of illustration and explanation only and not by way of limitation since various changes therein may be made by those skilled in the art without departing from the true spirit and scope of the present invention.

Our invention, together with further objects and advantages thereof will be better understood from a consideration of the following description taken in connection with the accompanying drawing in which:

The FIGURE is a flowsheet generally illustrating the principles of our new and novel process which results in the unique urea-ammonium polyphosphate granules having the novel properties mentioned above.

Referring now more specifically to the FIGURE, streams of concentrated urea solution 1 (produced in equipment not shown but well known to those skilled in the art) and ammonium polyphosphate melt 2 are continuously fed with recirculating load of undersize product granules 3 into pugmill granulator 4 which imparts a tumbling, kneading, and mixing action to the feed materials, the location of the urea inlet being upstream from, and separated by a minimum of 12 inches from the location of the melt inlet. Granulator discharge stream 5 is withdrawn and fed to cooler 6, typically a rotating-kiln-type cooler where the hot granules are contacted with air stream 7 to cool the granules, thus removing the heat imported to the granules by the streams of urea 1 and ammonium polyphosphate 2. The

stream of cooled granules 8 is then passed to size classifier 9, typically a set of vibrating screens which separates the stream into an oversize fraction, a product fraction, and an undersize fraction. The stream of oversize granules 10 is fed to crusher 11; the crushed oversize materials 12 are combined with the undersize stream 13, and the resulting stream 3 is recycled to granulator 4. The product-size stream 14 is collected as product.

The phosphoric acid used in our process may be of either the electric-furnace type or the wet-process type, or any mixture thereof, and a wide variety of acid concentrations may be used. The main objective of our process is to produce ammonium polyphosphate melt containing from about 1 percent to about 98 percent of its P 0 as polyphosphates. Acid of merchant-grade concentration (50-58 percent P 0 with wet-process orthophosphoric acid and 50-69 percent P 0 with electric-furnace orthophosphoric acid) may be ammoniated by the process shown in U.S. Pat. No. 3,382,059, Getsinger, dated May 7, 1968, assigned to the assignee of the present invention. Also, acids in the superphosphoric acid range (68-70 percent P 0 with wetprocess acid, and 72-85 percent P 0 with electricfurnace acid) may be ammoniated in a closed vessel at atmospheric or elevated pressures according to Hignett et al, U.S. Pat. No. 3,336,127 and the parent patents thereof. Either freshly prepared ammonium polyphosphate melt or ammonium polyphosphate solid may be utilized.

As has been discussed earlier, one of the objects of our process is to provide a means of producing an ammonium polyphosphate-containing product. Electricfurnace superphosphoric acid of percent P 0 which is about the maximum concentration of superphosphoric acid taught possible (see The Canadian Journal-of Chemistry, volume 34, 1956, page 790), contains about 98 percent of its P 0 as polyphosphates. Wet-process superphosphoric acid of about 79 percent P 0 content would, depending upon congeneric impurity contents, contain a similar proportion of its P 0 polyphosphates. Gestinger teaches that melts containing ammonium polyphosphates can be produced directly from reacting ammonia and wet-process phosphoric acid.

Urea solutions of percent or greater concentration are preferred in our process. This is in contrast with the 70-85 percent urea solution preferred by R. D. Mit chell et al in U.S. Pat. application Ser. No. 649,750, filed June 28, 1967, assigned to an assignee of the present invention and now abandoned. However, solutions of somewhat lower concentrations may be utilized if, and only if, a dryer is installed in the process. However, since the dryer greatly increases the initial capital investment and production cost, we prefer that our urea solutions contain at least 95 percent urea by weight. It should also perhaps be pointed out that solid urea may be used in our process if one desires to pay the considerably higher and premium cost therefor and if the ammonium polyphosphate is applied as a hot melt. However, the great advantages of our process are more fully realized when urea solutions of at least 95 percent concentration are employed. Alternatively, it is possible to utilize solid ammonium polyphosphate with the urea solution.

While our invention relates primarily to the granulation of urea and ammonium polyphosphate melt, other nitrogen fertilizers such as ammonium nitrate or ammonium sulfate can easily be used instead of urea and have been successfully tested in pilot plant tests.

Following are pertinent points discovered in our work:

1. Separating the point of entry of the urea solution and the melt into the pugmill by at least 12 inches prevented their direct contact and avoided urea hydrolysis which results in foaming and gas evolution.

2. Spraying the urea solution into the granulator at a location upstream from the point of entry'of the ammo-,

nium polyphosphate melt allowed the melt to overcoat the urea, thereby preventing subsequent dusting of urea from the granule surfaces.

3. As the polyphosphate content of the molten ammonium polyphosphate is increased, the stickiness of the melt increases, and more mechanical action as shear is required to induce crystallization and subsequent granulation.

4. When wet-process phosphoric acid is utilized in production of the ammonium polyphosphate melt, more mechanical working is required for crystallization of the melt and a greater proportion of recycled fines is needed to control granule size and avoid overgranulation, as compared with utilization of 100 percent thermal acid, i.e., electric-furnace acid.

5. When granulating products of 100 percent urea, the products tended to be somewhat undersized and dusty. The addition of some molten ammonium polyphosphate improves granulation characteristics.

6. Spraying the urea solution into the granulator through a spray nozzle and adding molten ammonium polyphosphate through a perforated pipe gave improved granulation with less formation of oversize materials as compared to adding these materials as single streams.

7. With urea solution concentrations greater than about 95 percent, no drying of the product is required. Elimination of the drying step substantially decreases investment and operating costs for production.

In order that those skilled in the art may better understand how the present invention can be practiced, the following examples of processes we have used in the production of granular urea-ammonium polyphosphate materials employing a pugmill granulation system therefore are given bywayof illustration and not by way of limitation. It should also be noted that the following examples I-V follow closely the'ch-ronological order of development of our new and unique process, and we have incorporated therein substantial quantities of information in order that all those skilled in the art may be taught fully the results of our labor. Accordingly, before presenting these examples, we have outlined in Table A below the. acceptable and preferred ranges of variables in our process which, when taken in conjunction with the FIGURE of our process, will enable practice thereof.

TABLE A Production of Urea-Ammonium Polyphosphate by Prilling in Liquid Medium: Acceptable and Preferred Range of Variables Variables Limits Preferred Ammonium polyphosphate Polyphosphate content, l-98 10-65 of total P 0 Degree of ammoniation,

lb. NH lunit of P 0 2.5-9.5 4.5-7.5 Melt temperature, F. 270-600 300-450 Urea solution Concentration, by wt. -100 99-995 Temperature, F. 250-325 290-310 Pugmill mixer Temperature, F.

Recycled fines 70-180 100-150 Mill discharge -250 -230 Pounds of recycled fines: 0.5-10 I I 2-6 pound product Solid ammonium polyphosphate may be used also with urea solution. 2 Solid urea may be used also with ammonium polyphosphate melt.

The product from our process may be advantageously used in a variety of ways to prepare highanalysis liquid fertilizers, some of which methods are taught in I-lignett et al. US. Pat. No. 3,336,127, and in Bottai et al., 3,578,433 supra.

EXAMPLE I Granular crystallized ammonium polyphosphate product of the invention was produced in continuous operation under typical equilibrium feed rates, flows, and process conditions shown in table 1 below.

EXAMPLES II THROUGH V Similar data for production of granular urea, and 30-30-0, 36-18-0, and 22-44-0 grades of urea ammonium polyphosphate are given in tables 11, III, IV, and V, respectively, and shown below.

TABLE I Production of Granular Ammonium Polyphosphate Type phosphoric acid Thermal Wet-Process Screen analysis (wt.%)

Pugmill effluent TABLE l-Continued Production of Granular Ammonium Polyphosphate Type phosphoric acid Thermal Wet-Process +6 mesh 17.1 15.3 5 6+10 mesh 41.0 54.2 10 mesh 41.9 30.5

TABLE I1 10 Production of Granular Urea Feed rates to pugmill (1b./hr.)

Urea solution 968 Recycled solids 2700 Temperature (F.)

Urea solution 302 Recycled solids 107 Pugmill effluent 210 Cooler effluent 131 Composition (wt.%)

Urea solution Nitrogen 46.1 Water 0.8 Process product Nitrogen 46.4 Water 0.2 Screen analysis (wt. /0)

Pugmill effluent +6 mesh 15.8 6 +10 mesh 40.0 10 mesh 44.2

TABLE III Electric Wet- Type phosphoric acid furnace Mixed Process Nominal grade 30-30-0 30-30-0 28-28-0 Feed rates to pugmill (lb./hr.) 1 Ammonium polyphosphate melt 483 433 500 Urea solution 482 486 500 Recycled solids 5560 4340 5800 Temperature (F.) 4O

Ammonium polyphosphate melt 389 392 395 Urea solution 298 295 273 Recycled solids 13 3 135 Pugmill effluent 180 187 175 Coolereffluent 144 141 44 Composition (wt.

Ammonium polyphosphate melt 45 Nitrogen 14.8 14.8 12.0 Total P 0 62.1 61.6 56.9 Nonortho P 0 of total P 0 48.4 50.9 20.9 Urea solution Nitrogen 46.0 45.9 46.0 Process product Nitrogen 30.4 30.1 27.4 Total 120;, 31.0 31.7 31.7 Water 0.3 0 7 0.3 Screen analysis (wt.

Pugmill effluent +6 mesh 2.3 8.4 10.4 6 +10 mesh 63.4 44.5 32.3 mesh 34.3 47.1 57.3.

Mixture of electric-furnace and wet-process acids (about 81) percent electricfurnace acid).

TABLE IV Production of Urea-Ammonium Phospmate 2:1:0 Ratio Electric Wet- Type phosphoric acid furnace Mixed process Nominal grade 36-18-0 36-18-0 34-17-0 Feed rates to pugmill (lb./hr.)

Urea solution 960 1126 890 Ammonium polyphosphate melt 475 482 400 Recycled solids 5643 5520 6420 Temperature (F.) Ammonium polyphosphatc melt 391 392 421 Production of Urea Ammonium Phosphate 2: l :0 Ratio Electric I Wet- Tvpe phosphoric acid furnace Mixed process Urea solution 300 305 305 Recycled solids 129 136 133 Pugmill effluent 193 198 177 Cooler effluent 152 155 14 Composition (wt.

Ammonium polyphosphatc melt Nitrogen 15.4 14.5 12.1 Total P 0 62.4 61.3 58.1 Nonortho P 0 of total P 0 59.3 42.7 34.0 Urea solution Nitrogen 46.3 46.1 46.1 Process product Nitrogen 35.9 35.6 34.7 Total P 0 20.9 20.3 20.0 Water 0.3 0.3 0.7 Screen analysis (wt. 7!)

Pugmill effluent +6 mesh 1 1.1 11.0 6.6 6 +10 mesh 20.3 1 37.1 24.4

-'10 mesh Mixture of ClCClllC fUlZflilQEfltLl wet-process acids (about percent electricfurnace acid).

TABLE V rd t f rseA n g s 1. 2 ti Electric Wet Type phosphoric acid furnace Mixed Process Nominal grade 2244-0 2244-0 21-42-0 Feed rates to pugmill (1b./hr.)

Nonortho P 0 of total P 0 50.0

Urea solution a w Nitrogen Process product Nitrogen Total P 0 Water Screen analysis (wt.

+6 mesh 6 +10 mesh. 4 -10 mesh l 3 Mixture of electric-furnace and wet-process acids (about 80 percent electricfurnace acid). I 7 h 1 While we have shown and described particular embodiments of our invention, modifications and variations thereof will occur to those skilled in the art. We wish it to be understood therefore that the appended 'claims are intended to cover such modifications and variations which are within the true scope and spirit of our invention.

We claim; I

1. A process for the production of strong cyrstalline granular high-analysis urea-ammonium polyphosphates eminently suitable for fertilizer materials, which consists essentially of the steps of:

1. simultaneously adding, at a temperature in the range from about 270 to about 600F, a stream of molten ammonium polyphosphate containing from about 10 to about 65 percent of its P 0 values as polyphosphate and, at a temperature of about 250 to about 325 F, a stream of concentrated urea solution, said ureasolution ranging in concentration from about 95 to about 99.5 percent by weight and in proportions to one another such that the ratio of pounds of urea to pounds of ammonium polyphosphate is in the range from about 20:80 to about 80:20, into a pugmill mixer in the presence of latermentioned recycled undersize in such a manner that the points of entry of the urea and ammonium polyphosphate melt streams are separated upstream from one another by at least 12 inches, with the urea stream being introduced upstream from the introduction of said molten ammonium polyphosphates, said manner of arranging said urea stream point of entry upstream by at least 12 inches from said ammonium polyphosphate melt point of entry substantially eliminating sulisequent reaction of said urea with said ammonium polyphosphate; 2. maintaining the temperature of the material disfrom the product-size material; crushing said oversize material and combining with said undersize material for recycle to said step 1 supra at a recycle rate expressed as pounds of recycle fines:pound of product in the ratio of about 0.5 to about 10 and introduced into said pugmill in step (1) supra at a temperature in the range from about to about F. 

2. maintaining the temperature of the material discharged from said pugmill in the range from about 130*F to about 250*F, and discharging therefrom the resulting solid urea ammonium polyphosphate mixture into cooling means; and
 3. passing the resulting cooled mixture of urea and ammonium polyphosphate over screens to separate the oversize material and the undersize material from the product-size material; crushing said oversize material and combining with said undersize material for recycle to said step (1) supra at a recycle rate expressed as pounds of recycle fines:pound of product in the ratio of about 0.5 to about 10 and introduced into said pugmill in step (1) supra at a temperature in the range from about 70* to about 180*F. 